The hard-core property of the Frenkel exciton is explicitly taken into account in the interaction of conduction electrons with a polarizable medium. The excitonic polaron bandwidth is less narrowed than in the boson approximation, does not vanish for infinite coupling, and is temperature independent. One consequence is that at high temperature the polaron motion is not dominated by hopping, but still contains a contribution from band motion.

We consider a case, the far-ultraviolet spectrum of an ionic crystal, where the local-field effects on the optical properties can easily be understood. The results show how local-field effects modify the optical properties of solids and complicate their interpretation and comparison with theory.

Correlation energies for simple-cubic metallic hydrogen are calculated using random-phase-approximation (RPA) methods. Hartree-Fock Bloch functions for the real lattice, including those for excited bands, were used as zeroth-order states. About 60% of the RPA correlation energy originates from intraband excitations. The RPA correlation energy, including exchange, is estimated to be about - 0.024 hartree/electron near the Hartree-Fock equilibrium, leading to a total energy of about -0.490 hartree/atom.

By deriving the complete Lagrangian and Rayleigh-Gilbert dissipation functions for an enclosed domain we have obtained a set of equations for the radial and translational motion of the domain. In this Letter we specialize to the case $\frac{\mathrm{dH}}{\mathrm{dx}}=0\mathrm{}$ and discuss the equations which govern radial motion only. We show that the critical collapse radius is a dynamic quantity which varies considerably from the value given by magnetostatics.

We study the isospin-changing component of the total second-order radiative-correction operator in a special class of unified gauge theories, with strong interactions taken into account to all orders, and show that the contribution of the weak interactions amounts only to providing an effective cutoff to the electromagnetic contribution.

Hints from existing data suggest that deviations from scaling behavior should appear at momenta $\left|q^2\right|\gtrsim 20$ Ge${\mathrm{V}}^2$. We interpret the deviations as due to structure of the hadronic constituents at ${10}^{-15\mathrm{}}$ cm and make experimental predictions.

It is argued that the strongly interacting non-Abelian gauge mesons of a renormalizable Lagrangian theory with spontaneously broken vacuum symmetry lie on Regge trajectories. This Lagrangian is of particular interest since it is identical to one found by Gervais and Neveu from the zero-slope limit of a dual-resonance model. The exchange of scalar mesons in vector-vector scattering with coupling constants as given by the Higgs-Kibble mechanism is crucial for our result.

The effects of paramagnetic hyperfine structure can be used to explain the recoil-free fraction "anomaly" recently reported for ${\mathrm{Dy}}^{161}$ in molybdenum.

A method is proposed for obtaining very narrow-frequency tunable $\gamma$-ray resonances of emission and absorption in nuclear transitions. The method is based on frequency modulation of the nuclear transition in molecules vibrationally excited with a coherent laser wave and having the same tunable projection of the translational velocity on the direction of the wave propagation. Laser-induced nuclear resonances have widths ${10}^{-2\mathrm{}}$-${10}^{-4\mathrm{}}$ times the Doppler width of nuclear transitions and a tuning range of about ${10}^{-5\mathrm{}}$.

We discuss the two-stream interaction of relativistic electron beams with a plasma in the nonlinear regime supported by computer simulation experiments.

The anisotropic viscoelastic and heat-conductivity properties of nematic liquid crystals offer the possibility of obtaining thermal convection with very low thresholds in homeotropic films heated from above. We present experiments on $p-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{e}-p-n-\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}$ (MBBA) and a simplified two-dimensional calculation.

Both analytic theory and inhomogeneous plasma simulations are presented to show the nonlinear development of the Brillouin and Raman backscatter instabilities. Nonlinear fluid behavior predicts a backscatter energy on the order of the incident laser energy, but particle trapping and heating effects associated with the excited electrostatic wave can significantly reduce this.

Effects of large phase gradients on the Josephson relation have been estimated by a modification of the Aslamazov-Larkin theory. The results are applied to explain the Dayem-Wyatt effect.

Previous theories of off-diagonal disorder in the tight binding model of a binary alloy are found to be incorrect in the dilute limit. We treat this limit exactly, and discuss the extension to arbitrary impurity concentrations.

A variational approximation to the Ising model in a magnetic field ($T<~T_c$), based on the zero-field solution, yields an upper bound on the free energy, an approximate equation of state, and a lower bound on the magnetization, all having the correct critical indices. In two dimensions, these are numerically evaluated, and compared with the results of series expansions.

New drag-versus-speed measurements with a freely spinning steel sphere in a cavity-type permanent-magnet suspension confirm the up-to-date theories of residual drag (Coriolis, eddy-current, and relaxation effects). A small excessive drag is of particular interest for the discussion of Keith's gravitational radiation effect.

Measurements of the far-infrared background radiation were made with a balloon-borne radiometer at an altitude of 44 km. Equivalent blackbody temperatures of ${2.55}_{-0.45\mathrm{}}^{+0.25\mathrm{}}$, ${2.45}_{-1.05\mathrm{}}^{+0.45\mathrm{}}$, and ${2.75}_{-2.75\mathrm{}}^{+0.8\mathrm{}}$ °K were obtained for the background radiation in bandwidths extending from 1 to 11.5, 13.5, and 18.5 ${\mathrm{cm}}^{-1\mathrm{}}$. The total measured flux in the largest bandwidth is dominated by the atmospheric emission by ozone and water. The radiation of these atmospheric components was determined independently in a separate pair of radiometer spectral responses.

The blackbody form of the 2.7° cosmic microwave radiation appears to be inconsistent with variation of Planck's constant by as much as 10% since the epoch $z\sim 1000$.

An experiment has been carried out employing time-of-flight techniques to compare the velocity of propagation of short pulses of eV (visible) and GeV electromagnetic radiation. The results give a relative velocity difference $\frac{\left[c\right(\mathrm{GeV})-c(\mathrm{eV}\left)\right]}{c\left(\mathrm{eV}\right)}=(1.8\mathrm{}\pm 6)\times {10}^{-6\mathrm{}}$. The velocity of 11-GeV energy electrons $v_e$ is measured and found to be approximately equal to the velocity of visible light: $\frac{(c-v_e)}{c}=(-1.3\mathrm{}\pm 2.7)\times {10}^{-6\mathrm{}}$.

We have measured the relative energy dependence of the invariant cross section for the inclusive reaction $p+p\to p+X$ for $0.05<~\frac{{M_X}^2}{\mathbf{s}}<~0.22$ at $\mathbf{t}=-0.33\mathrm{} \mathrm{and} -0.45\mathrm{}$ Ge${\mathrm{V}}^2$. The energy range from $s=80\mathrm{}$ to 480 Ge${\mathrm{V}}^2$ was covered continuously by taking data during the acceleration ramp of the National Accelerator Laboratory machine. The data are compared with the diffractive-excitation and Regge models.